Method of determining fluoride ions



United States Patent METHOD 0F DETERMINING FLUORIDE TUNE David P. Bartell, Natrona Heights, Rudolph B. Fricioni,

Cheswick, and Margaret A. McMahon, Pittsburgh, Pa,

assignors to Allegheny lLudlum Steel (Iorporation,

Brackenridge, Pa, a corporation of Fennsylvania N0 Drawing. Filed .lan. 28, 1959, Ser. No. 73?,486

6 Claims. (Ci. 23-23@) This invention relates to a method of determining the amount of uncombined nitric and hydrofluoric acid in nitric-hydrofluoric pickling baths and relates to particular to a determination of the amount of hydrofluoric acid or active fluoride ions in such a bath.

Nitric-hydrofluoric pickling solutions, containing from about 10 to 40% HNG and up to about 10% HF, are well known in the metals industry, and are commonly employed to remove oxide scales from titanium and stainless steel strip material. It is necessary, particularly where continuous pickling is being conducted, to frequently ascertain the scale-removing ability of the bath in order to secure an evenly pickled product. In the past, the strength of the bath has been determined by titration and by conventional analytical determination of the amount of nitric acid and hydrofluoric acid present. The conventional determination involves determining the total fluoride content in order to determine the hydrofluoric acid present. Such means are ineffective in accurately determining the scaleremoving ability of the bath in that much of the fluoride content is inactivated by other ingredients in the bath. The hydrofluoric acid determination is misleading in that the pickling ability of the bath is actually dependent on the ratio of active fluoride ions to the available nitric acid.

The active fluoride ion content of the bath is supplied, not only by the hydrofluoric acid present, but also by fluoride compounds which form in the pickling solution during use, but which are readily ionized. Fluoride compounds are also formed that are not readily ionized, and these compounds reduce the activity of the bath but are measured and calculated as hydrofluoric acid when conventional means of analysis are employed.

A method has now been found whereby the hydrofluoric acid and active fluoride ions and their ratio to the nitric acidmay be easily and continuously ascertained. By employing our method the activity or pickling ability of such a bath may be easily determined, thus enabling one to adjust the strength of the ,bath frequently and making it possible to secure a uniform pickling action.

It is, therefore, the object of the present invention to provide a means of accurately determining the activity or pickling ability of a nitric-hydrofluoric acid pickling bath.

It is also an object of the present invention to provide a method to accurately determine the amount of active fluoride ions in a nitric-hydrofluoric acid metal treating bath.

A still further object of the present invention is to provide a method of determining the amount of active fluoride ions in a nitric-hydrofluoric acid bath by adding a predetermined quantity of such a bath to an iron-thiocyanate solution and observing the change in color intensity of the solution.

Other objects and advantageous features of the present invention will be obvious from the following description:

In general, the present invention relates to the method of determining the potential chemical activity of an aqueous nitric-hydrofluoric acid pickling bath by introducing a sample of the bath into an amber colored aqueous ironthiocyanate solution and observing the color change of the resultant mixture. For practical purposes, of course, an accurate determination cannot ordinarily be made visually but must be determined by employing a color measuring instrument such as a colorimeter. For extremely reliable 3,fl27,24fl Patented Mar. 27, 1962 and accurate measurements, comparisons are made with color changes of the aqueous iron-thiocyanate solution that occur when the solutions are mixed with nitric-hydrofluoric acid solutions of known strength and activity.

It has been found that active fluoride ions will bleach the color of an iron-thiocyanate solution. In the method of the present invention, a predetermined amount of the pickling solution is added to a known strength ferric thiccyanate solution and by measuring the change incolor caused by the bleaching action of the hydrofluoric acid and active fluoride ions on the thiocyanate solution, the amount of hydrofluoric acid or active fluoride ions present can be accurately determined. The total amount of active acid present is then determined by titration of total acidity employing potassium oxalate to prevent metal ion precipitation, and potassium nitrate to sharpen the end point. The amount of nitric acid may then be determined by subtracting the acidity caused by the hydrofluoric acid or active fluoride ions from the total acidity.

Although color change or bleaching of the aqueous amber colored iron-thiocyanate solution may be observed visually if a dark colored solution is employed and the quantity of hydrofluoric acid of the pickling solution is large, practical use of the present method requires an instrument capable of accurately measuring slight color variations. Such instruments are well known and usually embody the principle of passing a beam of light through the colored solution onto photoelectric cells which, in turn, effects an electric current output to vary current flow through a galvonometer that is provided with a needle and dial to give comparative readings. The actual reading is frequently an arbitrary one obtained from a dial attached to a knob which varies electrical resistance by means of a potentiometer to correct or zero the galvanometer to a reading obtained for a given light intensity when a clear solution is employed in place of a colored solution. U.S. Patent No. 2,193,437 to Summerson shows such an instrument as may be employed in the method of the present invention. This patent clearly illustrates both the instrument and its use so that such details need not be considered in the present specification. To obtain precise measurements, it is necessary to zero the instrument on a clear base liquid (in the present instance distilled water) then establish potentiometer readings on several nitric-hydrofluoric acid solutions of known strength. A reading is then taken on the unknown. The amount of active fluoride ion content or hydrofluoric acid may be obtained by comparing such values.

The strength of the iron-thiocyanate concentration in the solution employed is unimportant except that there must be a suflicient amount to react with all of the active fluoride ions present in the pickling solution. However, colorimeters, such as described in U.S. Patent 2,193,437, are capable of detecting color and color changes where such colors are visualiy unobservable. Therefore, a mere trace of the iron-thiocyanate may be sufflcient to impart the desired color. On the other hand, the intensity of the color is limited only by the high range sensitivity of the colorimeter, so that the concentration may vary from a trace (preferably at least 1 gram per liter) to saturation of iron-thiocyanate in water. Best results may be obtained by intermediate strength solutions.

The iron-thiocyanate solution employed may be made up from any iron compound and thiocyanate compound that will release the desired ions to the solution. It has been found to be advantageous to dissolve ammonium thiocyanate and ferric chloride in water to obtain the de sired thiocyanate and iron content. A number of soluble thiocyanate or iron compounds may be employed for this purpose. Although the iron need not be entirely in the ferric state, it is necessary that ferric iron be present and preferably in an amount in excess of that necessary to combine with the thiocyanate to form ferric thiocyanate.

It has been found to be preferable to add some nitric acid to the test solution. Nitric acid serves to intensify the amber color of the solution and thus make color changes more easily ascertainable. However, excessive nitric acid concentrations result in unstable color conditions so that it would not be advisable to exceed about by weight, nitric acid in the thiocyanate test solution. The preferred range of nitric acid is about 4% to 9% by weight.

It has also been found preferable to employ lower concentrations of iron-thiocyanate where the nitric-hydrofluoric acid pickling Solutions normally contain fluoride contents Within the range of from about 0.1 to 1.5% and higher concentrations where the pickling solutions normally contain larger amounts of active fluorides, that is, 1.5% to 6%. Such ranges are referred to below in specific preferred solutions. These test solutions have been proven to render optimum results for low range solutions (containing 0.1 to 1.5% active fluoride) and high range solutions (containing 1.5 to 6% active fluoride).

(1) Low Range (Fluoride Content 0.1 to 1.5%)

To a one-liter volumetric flask containing approximately SOO-ml. of water, we add 30 ml. of nitric acid (specific gravity 1.42-l.3), 20 ml. of a solution of ammonium thiocyanate (3.806 grams to a liter of water) and 20 ml. of a ferric chloride solution (172 grams of ferric chloride to one liter of water). The flask is then filled to volume with water and allowed to stand for 10 to minutes prior to use.

(II) High Range (Fluoride Content 1.5 to 6%) To a one-liter volumetric flask containing approximately 500 ml. of water we add 60 ml. of nitric acid (specific gravity 1.42l.3), ml. of a solution of ammonium thiocyanate (7.612 grams to a liter of water) and 40 ml. of a ferric chloride solution (172 grams of ferric chloride to one liter of water). The flask is then filled to volume with water and allowed to stand for 10 to 15 minutes prior to use.

Recommended Procedure Twenty-five ml. of the appropriate hydrofluoric reagent are added to a 50 ml. volumetric flask. One ml. of the pickling solution is then added and the flask is filled to volume with water. After thorough mixing, the solution is transferred to an absorption cell and, using Water as a reference, the absorbance of the solution is obtained using a Klett Surnmerson Model 9003 Colorimeter, manufactured by Klett Manufacturing Company, New York, New York. The amount of active hydrofluoric acid is obtained by comparison of the absorbance with that of standard nitric-hydrofluoric acid solutions.

The above specific examples are given to illustrate the method of the present invention and in no way limit the invention to exact procedures set forth.

We claim:

1. The method of determining the active fluoride ion content of a nitric-hydrofluoric acid pickling bath which comprises, mixing a predetermined quantity of said pickling bath with a predetermined quantity of a test reagent that consists essentially of from about 1 gram per liter to saturation of ferric thiocyanate, an excess of ferric ions, up to 10% nitric acid, balance water, and measuring the color change in said reagent.

2. The method of determining the active fluoride ion content of a nitric-hydrofluoric acid pickling bath which comprises, mixing a predetermined quantity of said pickling bath with a predetermined quantity of a test reagent that consists essentially of from about 1 gram per liter to saturation of ferric thiocyanate, an excess of ferric ions, from 4 to 9% nitric acid, balance water, and measuring the color change in said reagent.

I 3. The method of determining the active fluoride ion content of a nitric-hydrofluoric pickling bath that contains from about 0.1% by Weight to about 1.5% by Weight fluoride content, which comprises, adding one milliliter of said bath to 25 milliliters of a reagent that consists of 4.26%, by Weight, HNO 0.0076%, by Weight, NH CNS, 0.344%, by weight, FeCl and balance to 50 ml. volume with water and measuring the color change in said reagent by inserting said mixture into a colorimeter, measuring the change in the color of said solution and comparing the results with colorimeter readings of nitric-hydrofluoric acid solutions of known strength active fluoride content.

4. The method of determining the active fluoride ion content of a nitric-hydrofluoric pickling bath that contains from about 1.5%, by weight to 6%, by weight fluoride content, which comprises, mixing one milliliter of said bath with 25 milliliters of a reagent that consists of 8.52%, by weight, HNO 0.0152%, by weight, NH CNS, 0.688%, by Weight, FeCl and balance to 50 ml. volume with water and measuring the color change in said reagent by inserting said mixture into a colorimeter, measuring the change in the color of said solution and comparing the results With colorimeter readings of nitric-hydrofluoric acid solutions of known strength active fluoride content.

5. The method of determining the active fluoride ion content of a nitric-hydrofluoric acid pickling bath which comprises, mixing a predetermined quantity of said pickling bath with a predetermined quantity of an aqueous test reagent that contains from a trace amount to saturation of ferric thiocyanate and an excess of ferric ions and measuring the color change of said reagent.

6. The method of determining the active fluoride ion content of a nitric-hydrofluoric acid pickling bath which comprises, mixing a predetermined quantity of said pickling bath with a predetermined quantity of test reagent that consists essentially of from about 1 gram per liter to saturation of ferric thiocyanate, an excess of ferric ions, up to 10% nitric acid, balance water, and measuring the color change in said reagent.

References Cited in the file of this patent Ingols: Analytical Chemistry, vol. 22, June 1950, pages 799-803.

Foster: Ibid., vol. 5, 1933, pages 234-236.

Snell: Col. Meth. of Anal., 1949 ed., page 750. 

4. THE METHOD OF DETERMINING THE ACTIVE FLUORIDE ION CONTENT OF A NITRIC-HYDROFLUORIC PICKLING BATH THAT CONTAINS FROM ABOUT 1.5%, BY WEIGHT TO 6%, BY WEIGHT FLUORIDE CONTENT, WHICH COMPRISES, MIXING ONE MILLILITER OF SAID BATH WITH 25 MILLILITERS OF A REAGENT THAT CONSISTS OF 8.52%, BY WEIGHT, HNO3, 0.0152%, BY WEIGHT, NH4CHNS,0.688%, BY WEIGHT, FECL3 AND BALANCE TO 50 ML. VOLUME WITH WATER AND MEASURING THE COLOR CHANGE IN SAID REAGENT BY INSERTING SAID MIXTURE INTO A COLORIMETER, MEASURING THE CHANGE IN THE COLOR OF SAID SOLUTION AND COMPARING THE RESULTS WITH COLORIMETER READINGS OF NITRIC-HYDROFLUORIC ACID SOLUTIONS OF KNOWN STRENGTH ACTIVE FLUORIDE CONTENT. 